An analytical method which can be used to determine the concentration of a wide range of chemical elements, using the intensity of their fluorescent X-rays. An X-ray beam is used to excite atoms in a sample; electrons near the nucleus emit secondary or fluorescent X-rays on reversion to their original states. Short-wavelength X-rays are sorted by diffraction in a pure analysing crystal of known d-spacing (see covalent radius). Since nλ‎ = 2d sinθ‎ (see bragg equation), θ‎ can be set to a value and radiation detected for a unique wavelength characteristic of the element being analysed. The intensity of the radiation measured, relative to a standard, is proportional to the concentration of the element. It is an important technique in the geochemical analysis of rocks.